AAT board members have approved a second shortlist - consisting of six contenders - following a decision last month to restart the pitching process.
Board members threw out an initial list of five firms, questioning the screening process before dismissing the entire proposal as short on detail.
Three of the six firms on the newly created shortlist - Lowe, Spa Advertising and Ogilvy PR Worldwide (which will jointly bid alongside Kith & Kin Communications) - were also on the initial shortlist.
The remaining three on the shortlist are Hermes Communication, Integrated Partner and SC Matchbox.
The winner, which will be revealed later this month, will organise a publicity campaign in Thailand and overseas, as well as a public forum on the authority's privatisation.